Support environmental variables for dynamo db client

[psoch]

Following up on #611
I'm not sure how to take advantage of the fix above because DynamoDBClient is abstract and while I could create my own version of it I would also like to take advantage of all of the utilties in FeatureDynamoDBClient. Maybe I'm missing something, but do you have any recommendations for using environmental variables with ff4j? Ideally I'd like to use something like spring boot properties where I could either have an environmental profile or dynamically update property values at runtime, but this doesn't seem possible with ff4j since the file name is hardcoded and we can't add overrides in most places. Thank you!